1. Field of the Invention
The invention relates generally to the field of marine seismic vibrators. More specifically, the invention relates to method for operating arrays of such marine vibrators to enhance low frequency content of the array signal output.
2. Background Art
Vibrator-type seismic energy sources known in the art impart seismic energy into the Earth's subsurface by moving a radiating surface in a particular manner. See, for example, U.S. Pat. No. 3,863,202 issued to Landrum, Jr. In one type of implementation, the radiating surface is coupled to a control system including a hydraulic ram and a control valve that selectively applies hydraulic pressure to each side of the hydraulic ram. The control valve is typically electrically operated. Electrical signals applied to the control valve generally correspond to the vibratory waveform that it is intended to be produced by the motion of the radiating surface. In order for the motion of the radiating surface to be efficiently coupled to the Earth, it is necessary to provide a large reactive mass coupled to the hydraulic ram opposite the radiating surface. In another implementation, a diaphragm placed in a body of water is moved in a similar manner through either electrical or electromechanical means.
A typical marine vibrator is illustrated and described in U.S. Pat. No. 3,349,367 issued to Wisotsky. Such vibrators comprise a sonic radiator driven by a hydraulic ram. The hydraulic pressures are derived from a surface source and applied by way of high pressure hoses to the hydraulic ram under control of a servo valve to effect movement of the sonic radiator over a predetermined frequency range. The vibrator is programmed through control signals to generate energy in the seismic frequency band between 10 and 190 Hz. In conducting the operations the vibrator output is swept through a range of frequencies as above noted either in an upsweep or downsweep. The inertial mass for the vibrator is provided by the structure housing the hydraulic ram and sonic radiator. Accordingly the housing such as that shown in FIG. 5 of the Wistosky '367 patent will vibrate at the same frequency as the sonic radiator and these vibrations are transmitted to any structure mounted on the housing, for example, a structure used to connect the marine vibrator to surface supporting and towing devices as well as to any equipment mounted to the structure near the vibrator. Another marine vibrator is described in U.S. Pat. No. 4,635,747 issued to Bird, Sr. et al.
A particular limitation to vibrator seismic sources known in the art relates to generating low frequency seismic energy, typically less than about 8 Hz. For such low frequencies, the reactive mass or diaphragm must be relatively large, and the amount of motion that must be imparted to the radiating surface is also relatively large. Controlling such motion so that it faithfully corresponds to the electrical control signal has also proven to be difficult.
Accordingly, there continues to be a need for marine seismic vibrator systems that provide sufficient low frequency energy for seismic surveying.